Re: [PATCH 1/2] block: ignore underlying non-stack devices io_opt

[Christoph Hellwig <hch@infradead.org>]

On Mon, Aug 18, 2025 at 02:31:20PM +0800, Yu Kuai wrote:
> Hi,
> 
> 在 2025/08/18 14:18, Christoph Hellwig 写道:
> > On Mon, Aug 18, 2025 at 02:14:06PM +0800, Yu Kuai wrote:
> > > Please take a look at the first patch, nothing special, the new flag
> > > will be passed to the top device.
> > 
> > But passing it on will be incorrect in many cases, e.g. for any
> > write caching solution.  And that is a much more common use case
> > than stacking different raid level using block layer stacking.
> 
> I don't quite understand why it's incorrect for write caching solution,
> can you please explain in details? AFAIK, the behaviour is only changed
> for the first mdraid device is the stacking chain.

The way I read the patch, the flag is inherited if any underlying
device sets it.

Now if you stack something that buffers most I/O the md raid limits
aren't really that relevant, and you'd rather expose the limits
for the writeback or read caching.